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Topic: MC-5 wet tank check valve problem? (Read 7797 times)

First time I started and aired up the MC 5 this spring I discover that the high pressure relief valve on the wet tank releases periodically. Only when the compressor is pumping and only when the engine is at operating RPMs. Never happens at idle. I assume there is a check valve after that tank that might be not opening all the way when the rest of the bus systems need air from that wet tank. This tank is located ahead of the rear axle. I don't think it's the governor because the compressor cycles on and off. The pressure gauge never goes below 90 or above 120.

If it is ahead of the rear axle it is on the left side it is your Dry tank. Wet tank would be in the front wheel well on the rear wall of the bogie on the driver side and the Park tank would be on the passenger side. There is a check valve on the out going line from the air dryer as well as a purge valve on the bottom of it which can also stick open. Sounds like you found the pressure relief valve on the side of the air dryer. There is a E type filter just off the park tank and a regulator also.There is also a check valve at the compressor when stuck open will allow air back through it as well and makes it hard to build air. But it sounds like yours is building air ok.Might just try replacing your PRV and see if it stops. They are cheap.

Dave,Mine is a 5A with a single tank in back, two in the front wheel well. I think they changed this arrangement on later MCIs. This tank is oriented horizontally and has a manual cable pull drain valve. There's always condensation there. I drain it daily. The pressure relief "pop off" valve is actually on the side of the tank. There is no dryer other than a small cylindrical device about the size of an aerosol can. It acts as some type of filter and also has a drain valve on it.

So the compressor is plumbed to the cylindrical device and from there directly to the rear tank. I have not looked at the schematic recently but I'm guessing there is a check valve separating this first tank in the system from the two tanks up front. If so, I wonder where it is located.

You are correct it builds air and will hold air over night. 30 pounds for several days.

Given the way that air pressure works, and the fact that your gauge reads normally, I would suspect that the safety valve is simply popping off too early. A check valve will tend to fail open, not closed, and won't tend to act as a regulator that only passes a reduced pressure of air - tends to be all or nothing, and you are getting something. If you had a 5C then I would tell you that there is a check valve located on the dry tank on the line from the wet tank, but if your dry tank is in the back then you need to check your schematic since mine is not valid for your system.

This bus has what the MCI manual refers to as a compressor filter. I have a gauge temporarily installed there. When the compressor is pumping and the engine is on high idle, the gauge will read up to 150 lbs at which point the pop off valve on the rear tank releases. Pressure goes down to 125 and then when it gets up to 150 the pop off releases again. The pressure gauge on the dash just goes between 90 and 120 never higher. When the compressor unloads the pop off quits releasing and the pressure on my temporary gauge reads about 120.

It seems like there is a restriction between that rear wet tank and the point in the system where the dash mounted gauge is attached. The governor must monitor pressure someplace near where the dashboard gauge is attached. Because it appears to be functioning normally.

A visual inspection of the rear tank does not show any check valves or regulators. Just one big copper line in and one big copper line back out.

Neither my parts manual nor the service manual show the air system schematic.

My understanding from reading this board forever, it seems, is that on that generation of MCI the dash gauge reads the accessory system, and the governor is fed from the accessory system so it's logical that they are both working in sync. The typical things in the system flow are air dryer, wet tank, dry tank, E-filter, pressure protection valve, then the accessory and parking brake systems from there. The check valve would be on the dry tank input from the wet tank, per my schematic, so you could look there. Where is your dry tank at, anyway? At this point a likely suspect is the pressure protection valve sticking, it is designed to be an on/off regulator of sorts.

OK here is my theory. I consulted the schematic and got the bus up on the ramps. The compressor is connected to the wet tank on the rear bulkhead which has the pop off relief valve on it. The relief valve opens at 150psi and closes at 125psi. The wet tank is connected to the dry tank via a 3/4" copper line. The check valve is at the end of that 3/4" line coming into the dry tank. The dash air pressure gauge and the governor see the pressure after the dry tank. The dry tank and Parking tank are located on the front bulk head in the front wheel housing. So the governor and dash gauge are seeing between 90 and 120psi at all times, but the wet tank has 150psi before the relief valve opens and then lowers the pressure to 125. As soon as the governor sees 120psi it unloads the compressor and the relief valve on the wet tank stops opening. Everything is fine until the governor sees the pressure in the system drop below 90psi and the whole cycle starts again.

Seems like something is restricting the air flow between the wet tank and the dry tank; like maybe that check valve. I have attached a picture of the dry tank and the Parking tank in the front wheel housing. The check valve is on the front of the dry tank on the end of the 3/4" copper line.

Maybe it is just me, but from reading and re-reading your posts Fred, i don't see what the problem is. You can build air, everything seems to work and hold pressure, the only thing is the relief valve opens when at high rpm and not at idle. Sounds like it is working like it should to me.

The pressure in his wet tank is going up to 150 psi while the pressure in the accessory system where the gauge and the governor is will only see 120 psi. Two more ideas - one, put a gauge on the dry tank somewhere to see what it is reading, and two, put a gauge on the accessory tank to see what it is reading and if that matches the dash gauge. There would normally be a filter and a pressure protection valve between the dry tank and the accessory tank, and there is indeed a check valve on the input to the dry tank, so get that gauge on and read its pressure. Should be easy to put the gauge on the drain valve outlet.

Exactly my thought, hence the idea to put a gauge on the tanks and see what's in there. I'd probably do the accessory tank first, it's right there in the side of the bus and you could put a gauge in the drain valve opening super easy. And at the same time verify if there is indeed some kind of blockage.

Yah I attached a guage to the aux tank (accessible from the service door ahead of the driver's side front tire. It mirrors the guage on the dash. I can't really attach it easily to the dry tank unless I get the bus up on ramps and get it blocked. Also I stopped at C&J this afternoon and they said the check valve could possibly be sticking and it is usually easy to dis assemble and clean and then put back together. So I soaked the fittings with PB blaster and sometime this weekend when I have time; I'll drive it up on the ramps and block the body securely. Then let the air out of the dry tank and try to get the check valve out of there and clean it up.

When you get it up on the ramps I would just replace the check valve ( they are cheap). And clean out all those fittings to be sure. I still don't know why your dry tank is in the front with the park tank. Where is your wet tank? Do you have an air dryer at all?

Dave,The 5A (sn5401) had the wet tank horizontally mounted on the rear bulkhead near the drive axle. No dryer on my bus; none shown on the MCI schematics. I've read that the wet tank got moved up front in later years because the air had a chance to cool off on the up there? Less condensation maybe?

Anyway, I'm going to take that check valve apart this weekend. I could replace it I suppose, but the mechanics told me they would just clean it. I'll let you know what I find.Fred